The present disclosure relates to a fixing device which fixes a toner image on a sheet and an image forming apparatus including the fixing device.
An image forming apparatus, such as a copying machine and a printer, is provided with a fixing device which fixes a toner image transferred on a sheet to the sheet. A fixing type of the fixing device includes a roller fixing type which employs a heating roller. In the roller fixing type fixing device, a most effective means for improving an energy saving performance is to make the heating roller thin and to decrease its heat capacity. However, the thinner the thickness of the heating roller is, the lower a rigidity of the heating roller is. Then, if the heating roller has a thickness of 0.5 mm or below, high rigid material, such as stainless alloy and iron, has been employed as a substrate layer of the heating roller.
However, such high rigid material including stainless alloy and iron has a heat conductive property lower than aluminum which is conventionally employed as the substrate layer. Thereby, a non-sheet passing area outside a sheet passing area is hardly cooled.
Then, a high heat conductive layer having a heat conductivity higher than the substrate layer may be provided around an outer circumferential face of the substrate layer. Because heat is transferred through the high heat conductive layer in a width direction, a temperature of the heating roller becomes equal in the width direction.
However, in a case where the high heat conductive layer is provided, if a plurality of the sheets are continuously fed and the non-sheet passing area is excessively heated, it is difficult to quickly transfer the heat of the non-sheet passing area.